Case hardened self-drilling, self-piercing and self-tapping screws are often used to reduce fastener joint complexity and fastener assembly. Self-drilling screws have point and thread configurations that allow the screws to cut threads into the mating materials. Self-piercing screws have point configurations that allow the screws to form their own pilot holes and then tap or cut threads into a mating component. Self-tapping screws, also referred to as thread forming screws, have point and thread configurations that allow the screws to form threads in mating components. These screws simplify the assembly process and provide economic benefits by eliminating the need for pre-drilled and pre-threaded holes, by helping to align fastener joints during assembly, and by reduced fastener joint packaging space.
There are many industrial applications where case hardened fasteners can be used to provide economic cost savings. A common usage of case hardened fasteners is in the assembly of thin sheet metal components such as the mounting of body panels to the frame of vehicles and the assembly of paneling to the structural components of a building. Case hardened screws are used in many industries such as appliance, automotive, aerospace, as well as others.
In order for self-tapping, self-drilling and self-piercing fasteners to perform the their intended functions, the surface hardness of the threads must be harder than the materials into which threads are being cut or formed. Typically, case hardened fasteners have been manufactured from steel which is case hardened by carbonnitriding or, less commonly, gas carburizing. This produces a fastener with a hardened steel surface, referred to as the "case," and a less hard, more ductile core. The hardened surface extends to a specified depth, which depends on the diameter of the fastener, and typically has a hardness greater than 45 HRC. The core hardness typically ranges from 28 to 39 HRC.
Many of the above applications for case hardened fasteners involve long term exposure to corrosive environments. The process for making self-tapping metal screws, in particular, customarily includes the step of plating the screws with a corrosion resistant barrier or sacrificial metal layer coatings after the hardening operation. Prior to plating, fasteners are cleaned of heat treat scale, oil and contaminants using either acid or caustic cleaning baths. The type of coating and the process in which a coating is applied depends on the desired corrosion resistance, cosmetic appearance, electrical conductivity, and friction characteristics. Coatings can be applied after cleaning by one of several methods including, but not limited to electroplating, mechanical plating, dip-spinning, and spraying.
Unfortunately, when hardened steel is either acid cleaned or electroplated it can become embrittled through a process called "hydrogen embrittlement." Hydrogen embrittlement is a process of time dependent subcritical crack formation and crack growth resulting from the cooperative interaction between static stress and hydrogen. The susceptibility of steel to hydrogen embrittlement has typically been related to increasing hardness, stress and the amount of hydrogen available for diffusion to tri-axial stresses. Case hardened fasteners are particularly susceptible to hydrogen embrittlement due to their high surface hardness and their processing under traditional manufacturing methods. The formation and growth of cracks due to hydrogen embrittlement typically result in the separation of the head of the fastener from the shank or threads and can occur within minutes or days after assembly.
In order to relieve the hydrogen embrittlement and reduce the danger of cracking, standard specifications call for electroplated fasteners to be baked, heat treated for 4 to 24 hours at 400.degree. F. within one hour of electroplating. Baking is not always completely successful in relieving the hydrogen embrittlement and adds cost to fasteners. A more certain method for preventing hydrogen embrittlement is therefore needed.
Hardened steel fasteners that are susceptible to hydrogen embrittlement are also susceptible to "stress corrosion cracking", also referred to as "environmentally assisted hydrogen embrittlement". Stress corrosion cracking of fasteners is similar to hydrogen embrittlement in that hydrogen is involved in embrittling the steel. However, in stress corrosion cracking, hydrogen is supplied by the corrosion reaction between the steel surface, the sacrificial coating and the environment. As with hydrogen embrittlement, fastener failure occurs some time after assembly and can vary from minutes to any time during the lifetime of the fastener. There are no known methods of relieving stress corrosion cracking susceptibility.
Soft steels are typically not as susceptible to hydrogen embrittlement. Unfortunately, soft steels do not contain a sufficient hardness to cut, pierce and form threads as self-drilling, self-piercing, or self-tapping fasteners. Any solution to prevent hydrogen embrittlement and stress corrosion cracking in self-tapping, self-piercing and self-drilling screws must address material susceptibility yet preserve the ability of the fasteners to perform their intended functions. Baking, choosing alternate coatings, and attempting to manage the fastener stress state have proved to be unreliable solutions to hydrogen embrittlement and stress corrosion cracking. The invention below describes a process and recommended materials in which self-tapping, self-piercing and self-drilling screws can be manufactured that are resistant to hydrogen embrittlement and stress corrosion cracking, yet can still perform their intended functions.